Abstract: :
Purpose: To study the effects of increasing stimulus size onthe response properties of retinal ganglion cells in the mouse.Methods: Single-unit responses were recorded from optic nervefibers of anesthetized mice in vivo. For each cell the receptivefield center was mapped on a tangent screen, using small spotsgenerated by a handheld ophthalmoscope. Area response functionswere then obtained by presenting computer-generated flashingspots whose diameter varied from 1 to 50 deg., presented ona uniform background of 25 cd/m2. Full-field illumination was50x80 deg. The spot was turned on (50 cd/m2) and off (0 cd/m2)at 0.1 Hz. Each spot diameter was presented eight times. AveragePSTHs were accumulated over all trials at each stimulus size.The center sign of the response (ON, OFF, ON/OFF), the responseamplitude and its duration were measured from the average PSTHand a transient/sustained index was computed.Results: Usingour hand-held ophthalmoscope and small diameter computer-generatedspots, we have encountered approximately equal proportions ofON- and OFF- center ganglion cells. Using the same stimulationconditions, no ON/OFF-center cells have been encountered. Themajority of both ON- and OFF-center cells responded optimallyto a stimulus whose diameter was between 4-21 deg, and theirresponses were attenuated when both smaller and larger diameterstimuli were centered on the receptive field. Increasing tofull-field illumination produced a significant decrease in thepeak firing rate of most cells by an average factor of 0.55±0.07(±SEM, p less than 0.001), indicating the presence ofan inhibitory surround. In contrast, increasing the stimulussize had little or no effect on the duration of the responseor on its transient/sustained index. In ~10% of the OFF-centercells, full-field illumination elicited an ON/OFF response,probably due to the presence of strong center–surroundinteractions. In another 10% of the cells the center responsewas completely suppressed by full-field stimulation.Conclusions:These data suggest that in the mouse, as in other mammalianretinas, center-surround receptive field interactions are importantin shaping of RGC response. Thus, full-field stimulation cannotbe used to accurately predict either the center sign or thepeak response for many cells.